The present invention relates to a system for controlling the transmission ratio of an infinitely variable belt-drive transmission for a vehicle, and more particularly to a control system in which low (L) range and drive (D) range can be automatically selected in accordance with the depression of an accelerator pedal of the vehicle.
Japanese Patent Laid-open No. 57-33255 discloses a control system of this type, in which operation range of an infinitely variable belt-drive transmission, such as parking (P-range), reverse (R-range), neutral (N-range), drive (D-range) and low (L-range), can be selected by the actuation of a shift lever like an ordinary automatic transmission device. In the D-range, transmission ratio is controlled throughout the whole area from the highest transmission ratio to the lowest ratio, and in the L-range, the transmission is downshifted to a higher ratio. Thus, when the L-range is selected, engine speed is quickly increased by depressing an accelerator pedal. However, releasing the accelerator pedal in the L-range apts to cause very high engine braking effect. Therefore, it is desirable to shift the shift lever quickly to the D-range before releasing the accelerator pedal after the acceleration at the L-range. Such quick operation of the shift lever may be difficult for the driver, especially for beginners.
On the other hand in the infinitely variable belt-drive transmission, the smaller the throttle valve opening degree is, the higher the transmission ratio is provided. Accordingly, if the accelerator pedal is depressed at the D-range, a large downshifting for rapid acceleration can not be effected. Further, because of the construction of the infinitely variable belt-drive transmission, the friction torque between a belt and pulleys becomes larger as the transmission is downshifted. Consequently, the response to the demand for rapid acceleration is low at the D-range.